The present invention relates to an electrode for charging a continuously running belt-like support (hereinafter referred to as "a web") to produce a static field used when coating a web with any of various kinds of liquid compositions in manufacturing photosensitized material film, photographic print paper, magnetic recording tapes, adhesive tape, pressure-sensitive paper, heat-sensitive paper, photosensitised printing plate, etc.
A coating system useful for coating various liquid compositions (hereinafter referred to as "a coating liquid") onto a web using a static field is described in Japanese Patent Publication No. Sho. 49-7050 and Japanese Laid-Open Patent Application No. Sho. 55-142565.
In this system, the static field is used to improve the adhesion between the web and the coating liquid. For this purpose, a web charging electrode is disposed upstream of the coating point adjacent the moving web. With this system, a variety of improvements in the coating process are realized, for example, easy starting of high speed coating and better protection of thick coatings. Furthermore, the system can be used with many different types of coating systems.
However, a problem arises with respect to a curtain coating method, which is required to perform coating at a very high speed. In a curtain coating method for a web of width not less than one meter and which is charged with a unipolar electric charge, there is generally employed a corona discharge apparatus which is provided with a set of electrically conductive wires acting as a corona discharge electrode and a rotatable roller supporting the web and acting as a grounding electrode. This apparatus has various problems caused by potential and charge irregularities over the surface of the web. Specifically, an irregular thickness (step unevenness) of the coating liquid is caused by longitudinal (transfer direction of the web) voltage irregularities, and liquid exhaustion tends to occur at the edge portions of the web due to an irregular voltage distribution in the width direction of the web as a result of a difference of the amounts of discharge between the center of the wire electrode and the opposite ends of the wires of the electrode.